Buggy code:
public abstract class MarkupContainer extends Component { private static final long serialVersionUID = 1L ; private static final Logger log = LoggerFactory . getLogger ( MarkupContainer . class ) ; private Object children ; private transient MarkupStream markupStream ; public MarkupContainer ( final String id ) { } public MarkupContainer ( final String id , IModel < ? > model ) { } public MarkupContainer add ( final Component ... childs ) { } private MarkupContainer findChild ( final Component child ) { } public final MarkupContainer addOrReplace ( final Component ... childs ) { } public final boolean autoAdd ( final Component component , MarkupStream markupStream ) { } public final boolean contains ( final Component component , final boolean recurse ) { } @ Override public final Component get ( final String path ) { } public MarkupStream getAssociatedMarkupStream ( final boolean throwException ) { } public IMarkupFragment getAssociatedMarkup ( ) { } public final MarkupStream getMarkupStream ( ) { } public IMarkupFragment getMarkup ( final Component child ) { IMarkupFragment markup = getMarkup ( ) ; if ( markup == null ) { return null ; } if ( child == null ) { return markup ; } <START_BUG> markup = markup . find ( null , child . getId ( ) , 0 ) ; <END_BUG> if ( markup != null ) { return markup ; } if ( Character . isDigit ( child . getId ( ) . charAt ( 0 ) ) ) { String id = child . getId ( ) ; boolean miss = false ; for ( int i = 1 ; i < ( id . length ( ) ) ; i ++ ) { if ( ( Character . isDigit ( id . charAt ( i ) ) ) == false ) { miss = true ; break ; } } if ( miss == false ) { markup = getMarkup ( ) ; if ( MarkupContainer . log . isWarnEnabled ( ) ) { MarkupContainer . log . warn ( ( "1.4<seq2seq4repair_space>to<seq2seq4repair_space>1.5<seq2seq4repair_space>migration<seq2seq4repair_space>issue:<seq2seq4repair_space>your<seq2seq4repair_space>item<seq2seq4repair_space>component<seq2seq4repair_space>should<seq2seq4repair_space>be<seq2seq4repair_space>derived<seq2seq4repair_space>from<seq2seq4repair_space>AbstractItem.<seq2seq4repair_space>Item=" + ( child . toString ( ) ) ) ) ; } } } return markup ; } public String getMarkupType ( ) { } public void internalAdd ( final Component child ) { } public boolean isTransparentResolver ( ) { } public Iterator < ? extends Component > iterator ( ) { } public final Iterator < Component > iterator ( Comparator < Component > comparator ) { } public void remove ( final Component component ) { } public final void remove ( final String id ) { } public final void removeAll ( ) { } public final void renderAssociatedMarkup ( final String openTagName , final String exceptionMessage ) { } public final MarkupContainer replace ( final Component child ) { } @ Override public MarkupContainer setDefaultModel ( final IModel < ? > model ) { } public final int size ( ) { } @ Override public String toString ( ) { } @ Override public String toString ( final boolean detailed ) { } public final < S extends Component > Object visitChildren ( final Class < ? > clazz , final IVisitor < S > visitor ) { } public final Object visitChildren ( final IVisitor < Component > visitor ) { } private final void addedComponent ( final Component child ) { } private final void children_add ( final Component child ) { } public final Component get ( int index ) { } private final Component children_get ( int index ) { } private final Object postprocess ( Object object , boolean reconstruct , MarkupContainer parent , int index ) { } private final Object children_get ( int index , boolean reconstruct ) { } private final String getId ( Object object ) { } private final Component children_get ( final String id ) { } private final int children_indexOf ( Component child ) { } private final Component children_remove ( Component component ) { } private final Component children_remove ( int index ) { } private final Object children_set ( int index , Object child , boolean reconstruct ) { } private final Component children_set ( int index , Component child ) { } private final int children_size ( ) { } private final Component put ( final Component child ) { } private final void removedComponent ( final Component component ) { } protected final void renderNext ( final MarkupStream markupStream ) { } @ Override protected void onComponentTagBody ( final MarkupStream markupStream , final ComponentTag openTag ) { } @ Override protected void onRender ( ) { } private final void renderComponentTagBody ( final MarkupStream markupStream , final ComponentTag openTag ) { } protected final void renderAll ( final MarkupStream markupStream , final ComponentTag openTag ) { } @ Override protected final void setMarkupStream ( final MarkupStream markupStream ) { } private static class ComponentSourceEntry extends org . apache . wicket . ComponentSourceEntry { private ComponentSourceEntry ( MarkupContainer container , Component component , IComponentSource componentSource ) { } private static final long serialVersionUID = 1L ; @ Override protected void setChild ( MarkupContainer parent , int index , Component child ) { } } @ Override void removeChildren ( ) { } @ Override void detachChildren ( ) { } @ Override void internalMarkRendering ( boolean setRenderingFlag ) { } private Component [ ] copyChildren ( ) { } @ Override void onBeforeRenderChildren ( ) { } @ Override void onAfterRenderChildren ( ) { } public boolean hasAssociatedMarkup ( ) { } @ Override void setRenderAllowed ( ) { } private static class ChildList extends AbstractList < Object > implements IClusterable { private static final long serialVersionUID = - 7861580911447631127L ; private int size ; private Object [ ] childs ; public ChildList ( Object children ) { } @ Override public Object get ( int index ) { } @ Override public int size ( ) { }

Fix:
markup = markup . find ( child . getId ( ) , 0 ) ;